The invention relates to a hydraulic control valve assembly and particularly, although not exclusively, to one for use as a safety feature with a hydraulically actuated clutch of a vehicle such as an agricultural tractor.
Hydraulically actuated clutches are common in agricultural tractors and similar vehicles and machines. Without safety systems, however, there is the danger that the vehicle will be started with a gear engaged and that, as the hydraulic pressure builds up after the vehicles engine is started, the clutch will gradually be engaged and the vehicle will move off without the operator intending it to. Clearly such an occurrence could lead to damage to objects in the path of the vehicle or injury to people. It is therefore advantageous to incorporate a safety system which prevents the vehicle moving off without a positive action by the operator to permit it.
One such safety system is described in U.S. Pat. No. 4,396,099. The clutch control system described includes a shut-off valve which prevents the supply of pressurised fluid to the clutch until the clutch pedal has been depressed and released.
A further potential problem with hydraulically engaged clutches occurs when the hydraulic system of the vehicle fails and the supply pressure of the fluid falls. Once the supply pressure falls below a certain level the clutch will slip leading to overheating and damage.
It is an object of the present invention to provide an improved hydraulic control valve assembly which both prevents unintended engagement of a hydraulically activated apparatus and protects the apparatus from harm if the hydraulic supply pressure falls below a predetermined level.